The present invention relates to a phase shift mask and a method of producing the same. More particularly, the present invention relates to a phase shift mask which is used to produce VLSI (Very Large Scale Integrated) circuits, and which is easy to produce and capable of forming fine-line line patterns. The present invention also relates to a method of producing such a phase shift mask.
As one type of phase shift mask, a halftone phase shift mask is known. The halftone phase shift mask is useful to form fine-line patterns, but suffers from two problems as stated below:
(1) During transfer, a sub-peak of light intensity appears in the vicinity of a pattern which is desired to be formed on a wafer by exposure, causing the exposed pattern to be undesirably deformed. This problem is particularly seen in the vicinity of a large open pattern. For a large open pattern which can be satisfactorily resolved without using the phase shift lithography technique, the halftone phase shift mask is inferior to a conventional chromium mask in terms of transfer characteristics. PA1 (2) When transfer exposure is sequentially carried out on a wafer by using a stepper, a multiple-exposure region occurs on the wafer in which adjacent shots (each of which is an area where pattern transfer is effected by a single exposure operation) overlap each other. When it is desired to prevent the multiple-exposure region from being undesirably exposed to exposure light, in the case of the conventional chromium mask, the peripheral portion of the mask is formed as a masking pattern (black pattern) portion. By doing so, exposure light is completely blocked at the peripheral portion of the mask, and thus the multiple-exposure region is prevented from being undesirably exposed. However, in the case of the halftone phase shift mask, the masking pattern portion is also translucent. Therefore, the multiple-exposure region is undesirably exposed to exposure light by being repeatedly subjected to multiple exposure. PA1 (A) A method in which an ultra-fine repeated pattern, which is finer than the resolution limit, is disposed at a mask region where practically no exposure light should pass (see Japanese Patent Application Unexamined Publication (KOKAI) No. 6-175347). PA1 (B) A method in which a halftone phase shift film and a light-blocking film or a film capable of providing a high contrast are stacked, and after the whole stack structure has been processed into a predetermined pattern, the light-blocking film or the film capable of providing a high contrast is processed into a necessary pattern. PA1 (i) An overlying shifter type phase shift mask which is obtained by a process in which a light-blocking film is provided on the whole surface of a transparent substrate, and after the light-blocking film has been formed into a predetermined pattern by a photoengraving process, a phase shift film is provided on the whole surface and subjected to a photoengraving process. PA1 (ii) An underlying shifter type phase shift mask which is obtained by a process in which, after a phase shift film and a light-blocking film have been provided in the mentioned order on the whole surface of a transparent substrate, the light-blocking film is processed into a predetermined pattern, and then the phase shift film is patterned. PA1 (1) In the latter, a so-called optical waveguide effect is produced, and a complicated pattern data adjustment is required to cancel it. However, the former needs no pattern data adjustment. PA1 (2) In the former, a process concerning the phase shift film, which is a process unique to a phase shift mask, can be additionally carried out after the conventional chromium mask process, whereas, in the latter the phase shift film process must be carried out during the chromium mask process. Accordingly, the former can use the conventional light-blocking film process without any change, whereas the latter necessitates reviewing a part of the light-blocking film process because of the presence of the phase shift film.
In other words, the halftone phase shift mask indispensably needs to impart light-blocking properties to some region on its substrate from a practical point of view. Accordingly, either of the following two methods has heretofore been adopted:
There are other types of conventional phase shift masks, e.g. a LEVENSON type phase shift mask, in which a mask has on a transparent substrate at least a light-blocking film for forming a first pattern and a phase shift film for forming a second pattern. Such phase shift masks are known to be useful for forming fine-line patterns. As phase shift masks of this sort, the following two types are generally known:
A comparison between the two types of phase shift mask reveals that the former is more advantageous than the latter for the following reasons:
Incidentally, the above-described solution (A) for halftone phase shift masks has the advantage that a mask can be produced by a single lithography process. However, since the above-described repeated pattern must be exceedingly fine, it is extremely difficult to fabricate.
In the case of the solution (B), the pattern formation is easy, but it is essentially necessary to carry out photoengraving twice. Therefore, the process undesirably lengthens.
Among the LEVENSON and other similar types of phase shift mask, the overlying shifter type phase shift mask also suffers from the following disadvantages: When a phase shift film is formed over a patterned light-blocking film, the phase shift film cannot uniformly be formed by the influence of steps in the light-blocking film. Nonuniformity in the thickness of the phase shift film causes the amount of phase shift given to exposure light passing through the film to become nonuniform, thus markedly degrading the performance of the phase shift mask.
In order to eliminate nonuniformity in the thickness of the phase shift film and to obtain a phase shift mask having an accurately controlled phase angle, it is necessary to reduce the thickness of the light-blocking film and to minimize steps in the light-blocking film. However, if the thickness of the light-blocking film is reduced, the light-blocking performance of the film is degraded, and when step-and-repeat exposure is carried out, a multiple-exposure region, in which adjacent shots (each of which is an area where pattern transfer is effected by a single exposure operation) overlap each other, is undesirably exposed to exposure light, causing the contrast to be undesirably reduced. Therefore, it is difficult to solve the above-described problem by reducing the thickness of the light-blocking film.
As a light-blocking film, it is common to use a chromium film formed by sputtering. FIG. 15 shows the relationship between the thickness of a chromium film and the transmittance thereof. Light-blocking performance usually required is 0.1% or less in terms of transmittance. Thus, it will be understood that a light-blocking film is required to have a thickness of at least 60 nm. In general, a light-blocking film may be required to have an anti-reflective function. In this case, an anti-reflective film is needed in addition to the chromium film. Accordingly, it is common for the light-blocking film to have a thickness of 100 nm or more.
In general, when a phase shift film is formed over a light-blocking film pattern having steps, steps which are equal or close to those of the light-blocking film pattern also occur on the surface of the phase shift film. Further, it is generally observed that, as shown in FIG. 16, the surface of the phase shift film (shifter film) is tapered toward an opening portion of the light-blocking film, depending upon the type of method used for forming the phase shift film. In this case, the amount of phase shift given to exposure light becomes nonuniform at the opening portion, causing the effect of the phase shift mask to be reduced.
Phase shift films are generally formed by using a silicon oxide film. In a case where the silicon oxide film has a thickness variation of 60 nm, which is equal to the above-described chromium step height, in the opening portion, the variation of the phase shift is as large as about 30.degree. in the case of i-line lithography and nearly 45.degree. in the case of KrF excimer laser lithography. When the variation of the film thickness, including the anti-reflective film, is 100 nm, the phase shift variation is about 50.degree. in the i-line lithography and about 70.degree. in the KrF excimer laser lithography. A phase shift mask having such a large phase shift variation cannot be used in practical application.
Conversely, when the phase shift variation allowable range is assumed to be .+-.10.degree., the thickness variation allowable for the phase shift film of silicon oxide is only about 20 nm in the case of i-line lithography and about 14 nm in the case of KrF excimer laser lithography. Accordingly, unless a special film forming method is adopted, the allowable step height of the light-blocking film pattern becomes considerably smaller than the above-described step height 60 nm, although it depends on the type of shifter film forming method employed. Thus, the light-blocking performance is undesirably degraded, as will be clear from FIG. 15.